To supply delivery pressure oil from a hydraulic pump to a plurality of hydraulic actuators, a plurality of operating valves are provided in the delivery conduit of the hydraulic pump, pressure oil being supplied to the hydraulic actuators by switching these operating valves. A problem with this arrangement is that when supplying pressure oil simultaneously to the plurality of hydraulic actuators, the supply of pressure oil reaches only to those hydraulic actuators having small loads, with no pressure oil being supplied to those hydraulic actuators having large loads.
An example of a hydraulic circuit designed to be a solution of the above problem is disclosed in Japanese Patent Publication No. 2-49405.
FIG. 1 schematically shows such a hydraulic circuit. The hydraulic circuit shown includes operating valves 2 provided in a delivery conduit 1a of a hydraulic pump 1 and pressure compensating valves 5 provided in circuits 4 connecting the operating valves 2 to hydraulic actuators 3. Check valves 6 detect the maximum of the load pressures. The detected load pressure is caused to act on the pressure compensating valves 5 so as to set them to a pressure level counterbalancing this load pressure, thereby equalizing the pressures on the output sides of the operating valves 2. Thus, when the operating valves 2 are simultaneously operated, pressure oil can be supplied to the hydraulic actuators 3 in a flow dividing ratio proportional to the respective opening areas of the operating valves 2.
With such a hydraulic circuit, the pressure compensating valves 5 function so as to enable a flow division proportional to the respective opening areas of the operating valves 2 regardless of the magnitudes of the loads of the hydraulic actuators 3, so that it is possible for the hydraulic actuators 3 to be supplied with delivery pressure oil from a single hydraulic pump 1 in a flow dividing ratio proportional to the respective operation amounts of the operating valves 2.
However, due to the setting of the pressure compensating valves 5 to a pressure level counterbalancing the maximum load pressure, the opening (aperture) of the pressure compensating valve 5 having a relatively high load pressure is large, whereas the opening (aperture) of the pressure compensating valve 5 having a relatively low load pressure is small. Thus, when the difference in load pressure is excessively large, the opening of the pressure compensating valve 5 having a relatively low load pressure becomes extremely small, with the result that practically no pressure oil is supplied to the hydraulic actuator 3 under the lower load pressure. When applied to a swivel working machine, this hydraulic circuit involves the following problem: the swiveling body has a large inertial force, so that when the swiveling body and the working unit are simultaneously operated, the swiveling body has, in the early stage of swiveling, a load pressure which is substantially higher than that of the working unit cylinder, resulting in practically no pressure oil being supplied to the working unit cylinder. When the swiveling body starts to swivel at a steady speed, the load pressure of the swiveling hydraulic motor is lowered, with the result that a large amount of pressure oil is supplied to the working unit cylinder.
Thus, when the swiveling body and the working unit are simultaneously operated, the working unit performs practically no operation in the early stage of swiveling. The working unit starts to operate only when the swiveling body has started to operate at a steady speed.